前面在 EventBus設計與實現分析——特性介紹中介紹了EventBus的基本用法，及其提供的大多數特性的用法；在EventBus設計與實現分析——訂閱者的注冊 中介紹了EventBus中訂閱者注冊的過程。這里就繼續分析EventBus的代碼，來了解其事件發布的過程。

事件的發布

如我們前面已經了解到的，在EventBus中，有兩種不同類型得事件，一種是普通事件，事件被通知給訂閱者之后即被丟棄，另一種是Sticky事件，事件在被通知給訂閱者之后會被保存起來，下次有訂閱者注冊針對這種事件的訂閱時，訂閱者會直接得到通知。

在EventBus中，會以兩個不同的方法來發布這兩種不同類型的事件，這兩個方法分別是post(Object event)和postSticky(Object event)：

private final Map<Class<?>, Object> stickyEvents;private final ThreadLocal<PostingThreadState> currentPostingThreadState = new ThreadLocal<PostingThreadState>() {@Overrideprotected PostingThreadState initialValue() {return new PostingThreadState();}}; ....../** Posts the given event to the event bus. */public void post(Object event) {PostingThreadState postingState = currentPostingThreadState.get();List<Object> eventQueue = postingState.eventQueue;eventQueue.add(event);if (!postingState.isPosting) {postingState.isMainThread = Looper.getMainLooper() == Looper.myLooper();postingState.isPosting = true;if (postingState.canceled) {throw new EventBusException("Internal error. Abort state was not reset");}try {while (!eventQueue.isEmpty()) {postSingleEvent(eventQueue.remove(0), postingState);}} finally {postingState.isPosting = false;postingState.isMainThread = false;}}} ....../*** Posts the given event to the event bus and holds on to the event (because it is sticky). The most recent sticky* event of an event's type is kept in memory for future access by subscribers using {@link Subscribe#sticky()}.*/public void postSticky(Object event) {synchronized (stickyEvents) {stickyEvents.put(event.getClass(), event);}// Should be posted after it is putted, in case the subscriber wants to remove immediatelypost(event);} ....../** For ThreadLocal, much faster to set (and get multiple values). */final static class PostingThreadState {final List<Object> eventQueue = new ArrayList<Object>();boolean isPosting;boolean isMainThread;Subscription subscription;Object event;boolean canceled;}

postSticky()僅是在保存了事件之后調用post()來發布事件而已。而在post()中，會借助于PostingThreadState來執行事件發布的過程。PostingThreadState為發布的事件提供了排隊功能，同時它還描述一些發布的線程狀態。PostingThreadState還是發布過程跟外界交流的一個窗口，外部可通過EventBus類提供的一些方法來控制這個狀態，進而影響發布過程，比如取消發布等操作。PostingThreadState對象在ThreadLocal變量中保存，可見發布的事件的隊列是每個線程一個的。post()方法會逐個取出事件隊列中的每一個事件，調用postSingleEvent()方法來發布。

private void postSingleEvent(Object event, PostingThreadState postingState) throws Error {Class<?> eventClass = event.getClass();boolean subscriptionFound = false;if (eventInheritance) {List<Class<?>> eventTypes = lookupAllEventTypes(eventClass);int countTypes = eventTypes.size();for (int h = 0; h < countTypes; h++) {Class<?> clazz = eventTypes.get(h);subscriptionFound |= postSingleEventForEventType(event, postingState, clazz);}} else {subscriptionFound = postSingleEventForEventType(event, postingState, eventClass);}if (!subscriptionFound) {if (logNoSubscriberMessages) {Log.d(TAG, "No subscribers registered for event " + eventClass);}if (sendNoSubscriberEvent && eventClass != NoSubscriberEvent.class &&eventClass != SubscriberExceptionEvent.class) {post(new NoSubscriberEvent(this, event));}}}private boolean postSingleEventForEventType(Object event, PostingThreadState postingState, Class<?> eventClass) {CopyOnWriteArrayList<Subscription> subscriptions;synchronized (this) {subscriptions = subscriptionsByEventType.get(eventClass);}if (subscriptions != null && !subscriptions.isEmpty()) {for (Subscription subscription : subscriptions) {postingState.event = event;postingState.subscription = subscription;boolean aborted = false;try {postToSubscription(subscription, event, postingState.isMainThread);aborted = postingState.canceled;} finally {postingState.event = null;postingState.subscription = null;postingState.canceled = false;}if (aborted) {break;}}return true;}return false;}....../** Looks up all Class objects including super classes and interfaces. Should also work for interfaces. */private static List<Class<?>> lookupAllEventTypes(Class<?> eventClass) {synchronized (eventTypesCache) {List<Class<?>> eventTypes = eventTypesCache.get(eventClass);if (eventTypes == null) {eventTypes = new ArrayList<>();Class<?> clazz = eventClass;while (clazz != null) {eventTypes.add(clazz);addInterfaces(eventTypes, clazz.getInterfaces());clazz = clazz.getSuperclass();}eventTypesCache.put(eventClass, eventTypes);}return eventTypes;}}/** Recurses through super interfaces. */static void addInterfaces(List<Class<?>> eventTypes, Class<?>[] interfaces) {for (Class<?> interfaceClass : interfaces) {if (!eventTypes.contains(interfaceClass)) {eventTypes.add(interfaceClass);addInterfaces(eventTypes, interfaceClass.getInterfaces());}}}

postSingleEvent()要發布事件，首先需要找到訂閱者，我們前面在 訂閱者的注冊 中看到，訂閱者注冊時會在subscriptionsByEventType中保存事件類型和訂閱者的映射關系，那要找到訂閱者豈不是很容易？

其實不完全是。關鍵是對于事件類型的處理。要通知的事件類型的訂閱者不一定僅僅包含事件對象本身的類型的訂閱者，還可能要通知事件類型的父類或實現的接口的類型的訂閱者。在eventInheritance被置為true時，就需要通知事件類型的父類或實現的接口的類型的訂閱者。lookupAllEventTypes()和addInterfaces()就用于查找所有這樣的類型。

postSingleEvent()會逐個事件類型的去通知相應得訂閱者，這一任務由postSingleEventForEventType()來完成。而在postSingleEventForEventType()中則是根據subscriptionsByEventType找到所有的訂閱者方法，并通過postToSubscription方法來逐個的向這些訂閱者方法通知事件。

private void postToSubscription(Subscription subscription, Object event, boolean isMainThread) {switch (subscription.subscriberMethod.threadMode) {case POSTING:invokeSubscriber(subscription, event);break;case MAIN:if (isMainThread) {invokeSubscriber(subscription, event);} else {mainThreadPoster.enqueue(subscription, event);}break;case BACKGROUND:if (isMainThread) {backgroundPoster.enqueue(subscription, event);} else {invokeSubscriber(subscription, event);}break;case ASYNC:asyncPoster.enqueue(subscription, event);break;default:throw new IllegalStateException("Unknown thread mode: " + subscription.subscriberMethod.threadMode);}}....../*** Invokes the subscriber if the subscriptions is still active. Skipping subscriptions prevents race conditions* between {@link #unregister(Object)} and event delivery. Otherwise the event might be delivered after the* subscriber unregistered. This is particularly important for main thread delivery and registrations bound to the* live cycle of an Activity or Fragment.*/void invokeSubscriber(PendingPost pendingPost) {Object event = pendingPost.event;Subscription subscription = pendingPost.subscription;PendingPost.releasePendingPost(pendingPost);if (subscription.active) {invokeSubscriber(subscription, event);}}void invokeSubscriber(Subscription subscription, Object event) {try {subscription.subscriberMethod.method.invoke(subscription.subscriber, event);} catch (InvocationTargetException e) {handleSubscriberException(subscription, event, e.getCause());} catch (IllegalAccessException e) {throw new IllegalStateException("Unexpected exception", e);}}

在postToSubscription()中事件的通知又分為同步的通知和異步的通知。同步的通知是直接調用invokeSubscriber(Subscription subscription, Object event)方法，這會將事件對象傳遞給訂閱者方法進行調用。而異步的通知則是將事件及訂閱者拋給某個poster就結束。

對于某個訂閱者的通知要采用同步通知還是異步通知則需要根據訂閱者的ThreadMode及事件發布的線程來定。具體得規則為：
訂閱者的線程模式是POSTING --------------------------------> 同步通知
訂閱者的線程模式是MAIN + 事件發布線程是主線程 ---------------> 同步通知
訂閱者的線程模式是BACKGROUND + 事件發布線程不是主線程 ------> 同步通知
訂閱者的線程模式是BACKGROUND + 事件發布線程是主線程 --------> 異步通知
訂閱者的線程模式是MAIN + 事件發布線程不是主線程 --------------> 異步通知
訂閱者的線程模式是ASYNC ----------------------------------> 異步通知

同步通知和異步通知各三種。但三種異步通知本身又各不相同，它們分別由三種不同的Poster來處理，訂閱者的線程模式是BACKGROUND + 事件發布線程是主線程的異步通知由BackgroundPoster來處理，訂閱者的線程模式是MAIN + 事件發布線程不是主線程的異步通知由HandlerPoster來處理，而訂閱者的線程模式是ASYNC的異步通知由AsyncPoster來處理。

接著就來看一下這些Poster。首先是HandlerPoster：

package org.greenrobot.eventbus;import android.os.Handler; import android.os.Looper; import android.os.Message; import android.os.SystemClock;final class HandlerPoster extends Handler {private final PendingPostQueue queue;private final int maxMillisInsideHandleMessage;private final EventBus eventBus;private boolean handlerActive;HandlerPoster(EventBus eventBus, Looper looper, int maxMillisInsideHandleMessage) {super(looper);this.eventBus = eventBus;this.maxMillisInsideHandleMessage = maxMillisInsideHandleMessage;queue = new PendingPostQueue();}void enqueue(Subscription subscription, Object event) {PendingPost pendingPost = PendingPost.obtainPendingPost(subscription, event);synchronized (this) {queue.enqueue(pendingPost);if (!handlerActive) {handlerActive = true;if (!sendMessage(obtainMessage())) {throw new EventBusException("Could not send handler message");}}}}@Overridepublic void handleMessage(Message msg) {boolean rescheduled = false;try {long started = SystemClock.uptimeMillis();while (true) {PendingPost pendingPost = queue.poll();if (pendingPost == null) {synchronized (this) {// Check again, this time in synchronizedpendingPost = queue.poll();if (pendingPost == null) {handlerActive = false;return;}}}eventBus.invokeSubscriber(pendingPost);long timeInMethod = SystemClock.uptimeMillis() - started;if (timeInMethod >= maxMillisInsideHandleMessage) {if (!sendMessage(obtainMessage())) {throw new EventBusException("Could not send handler message");}rescheduled = true;return;}}} finally {handlerActive = rescheduled;}} }

這是一個Handler。其內部有一個PendingPostQueue queue，enqueue()操作即是用描述訂閱者方法的Subscription對象和事件對象構造一個PendingPost對象，然后將這個PendingPost對象放入queue中，并在Handler沒有在處理事件分發時發送一個消息來喚醒對于事件分發的處理。

而在handleMessage()中，則是逐個從queue中取出PendingPost對象，并通過EventBus的invokeSubscriber(PendingPost pendingPost)來傳遞事件對象調用訂閱者方法。這里調用的invokeSubscriber()方法與前面那個同步版本略有差異，它會將Subscription對象和事件對象從PendingPost對象中提取出來，并調用同步版的方法，同時還會釋放PendingPost對象。

這里有一個蠻巧妙得設計，就是那個maxMillisInsideHandleMessage，它用于限制一次事件發布所能消耗的最多的主線程時間。如果事件限制到了的時候訂閱者沒有通知完，則會發送一個消息，在下一輪中繼續處理。

這是一個典型的生產者-消費者模型，生產者是事件的發布者線程，而消費者則是主線程。

PendingPost對象是通過一個鏈表來組織的。

package org.greenrobot.eventbus;final class PendingPostQueue {private PendingPost head;private PendingPost tail;synchronized void enqueue(PendingPost pendingPost) {if (pendingPost == null) {throw new NullPointerException("null cannot be enqueued");}if (tail != null) {tail.next = pendingPost;tail = pendingPost;} else if (head == null) {head = tail = pendingPost;} else {throw new IllegalStateException("Head present, but no tail");}notifyAll();}synchronized PendingPost poll() {PendingPost pendingPost = head;if (head != null) {head = head.next;if (head == null) {tail = null;}}return pendingPost;}synchronized PendingPost poll(int maxMillisToWait) throws InterruptedException {if (head == null) {wait(maxMillisToWait);}return poll();}}

還有PendingPost：

package org.greenrobot.eventbus;import java.util.ArrayList; import java.util.List;final class PendingPost {private final static List<PendingPost> pendingPostPool = new ArrayList<PendingPost>();Object event;Subscription subscription;PendingPost next;private PendingPost(Object event, Subscription subscription) {this.event = event;this.subscription = subscription;}static PendingPost obtainPendingPost(Subscription subscription, Object event) {synchronized (pendingPostPool) {int size = pendingPostPool.size();if (size > 0) {PendingPost pendingPost = pendingPostPool.remove(size - 1);pendingPost.event = event;pendingPost.subscription = subscription;pendingPost.next = null;return pendingPost;}}return new PendingPost(event, subscription);}static void releasePendingPost(PendingPost pendingPost) {pendingPost.event = null;pendingPost.subscription = null;pendingPost.next = null;synchronized (pendingPostPool) {// Don't let the pool grow indefinitelyif (pendingPostPool.size() < 10000) {pendingPostPool.add(pendingPost);}}}}

PendingPostQueue是一個線程安全的鏈表，其中鏈表的節點是PendingPost，它提供了最最基本的入隊和出隊操作而已。PendingPost再次用了對象池，它提供了獲取對象和釋放對象的方法。EventBus的作者真的還是蠻喜歡用對象池的嘛。

然后再來看BackgroundPoster：

package org.greenrobot.eventbus;import android.util.Log;/*** Posts events in background.* * @author Markus*/ final class BackgroundPoster implements Runnable {private final PendingPostQueue queue;private final EventBus eventBus;private volatile boolean executorRunning;BackgroundPoster(EventBus eventBus) {this.eventBus = eventBus;queue = new PendingPostQueue();}public void enqueue(Subscription subscription, Object event) {PendingPost pendingPost = PendingPost.obtainPendingPost(subscription, event);synchronized (this) {queue.enqueue(pendingPost);if (!executorRunning) {executorRunning = true;eventBus.getExecutorService().execute(this);}}}@Overridepublic void run() {try {try {while (true) {PendingPost pendingPost = queue.poll(1000);if (pendingPost == null) {synchronized (this) {// Check again, this time in synchronizedpendingPost = queue.poll();if (pendingPost == null) {executorRunning = false;return;}}}eventBus.invokeSubscriber(pendingPost);}} catch (InterruptedException e) {Log.w("Event", Thread.currentThread().getName() + " was interruppted", e);}} finally {executorRunning = false;}}}

BackgroundPoster與HandlerPoster還是挺像的。兩者的差別在于BackgroundPoster是一個Runnable，它的enqueue()操作喚醒對于事件分發的處理的方法，是將對象本身放進EventBus的ExecutorService中執行來實現的；另外在處理事件分發的run()方法中，無需像HandlerPoster的handleMessage()方法那樣考慮時間限制，它會一次性的將隊列中所有的PendingPost處理完才結束。

對于某一個特定事件，一次性的將所有的PendingPost遞交給BackgroundPoster，因而大概率的它們會在同一個線程被通知。但如果訂閱者對事件的處理過快，在下一個PendingPost還沒來得及入隊時即執行結束，則還是有可能在不同的線程中被通知。

最后再來看一下AsyncPoster：

class AsyncPoster implements Runnable {private final PendingPostQueue queue;private final EventBus eventBus;AsyncPoster(EventBus eventBus) {this.eventBus = eventBus;queue = new PendingPostQueue();}public void enqueue(Subscription subscription, Object event) {PendingPost pendingPost = PendingPost.obtainPendingPost(subscription, event);queue.enqueue(pendingPost);eventBus.getExecutorService().execute(this);}@Overridepublic void run() {PendingPost pendingPost = queue.poll();if(pendingPost == null) {throw new IllegalStateException("No pending post available");}eventBus.invokeSubscriber(pendingPost);}}

它會對每一個通知（訂閱者方法 + 訂閱者對象 + 事件對象）都起一個不同的task來進行。

用一張圖來總結EventBus中事件通知的過程：

EventBus發布事件的過程大體如此。

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